Pacific Gas and Electric (PG&E) recently started the process of shutting down the Diablo Canyon generation facility, the last active nuclear power plant in California. The power plant, located near Avila Beach on the central Californian coast, consists of two 1,100 megawatt (MW) reactors and produces 18,000 gigawatt-hours (GWh) of electricity a year, about 8.5 percent of California’s electricity consumption in 2015. It has been, up until this point, the single largest electrical generation facility in the state.

Looming over the imminent closure of Diablo Canyon is California State legislative bill SB 350, or the Clean Energy and Pollution Reduction Act of 2015. The act is a cornerstone of the state’s ongoing efforts to decarbonize its electricity grid by requiring utilities to include renewable sources for a portion of their electrical generation in future years. The mandate also requires utilities to run programs designed to double the efficiency of electricity and natural gas consumption.

But a number of significant unanswered questions remain about this ambitious energy policy, as the planned closing by 2025 of Diablo Canyon illustrates. Can utilities supply electricity around the clock using these alternative generation sources? And crucially, can energy storage technologies provide the power on demand that traditional generators have done?

Moving away from nuclear power

Nuclear power plants saw their heyday in the early 1970s and were praised for their ability to produce large amounts of electricity at a constant rate without the use of fossil fuels.

However, due to negative opinion and costly renovations, we are now observing a trend whereby long-running nuclear power plants are shutting down and very few new plants are being scheduled for construction in the United States.

Utilities are moving toward renewable electricity generation, such as solar and wind, partially in response to market forces and partially in response to new regulations that require utilities to reduce greenhouse gas emissions. In California, in particular, the shift toward renewable energy for market and environmental reasons, along with the public’s negative perception of nuclear energy, has caused utilities to abandon nuclear power.

While opponents can view the shutdown of nuclear power plants as a health and environmental success, closing nuclear plants intensifies the challenges faced by utilities to meet electricity consumption demand while simultaneously reducing their carbon footprint. PG&E, for example, has pledged to increase renewable energy sources and energy efficiency efforts, but this alone will not help them supply their customers with electricity around the clock. What can be used to fill the sizable gap left by Diablo Canyon’s closing?

Solar power plants that use the sun’s heat to make electricity can store energy as molten salt.Department of Energy

Solar and wind energy sources are desirable as they produce carbon-free electricity without producing toxic and dangerous waste byproducts. However, they also suffer from the drawback of being able to produce electricity only intermittently throughout the day. Solar energy can be utilized only when the sun is out, and wind speeds vary unpredictably.

In order to meet customer electricity demand at all hours, energy storage technologies, alongside more renewable sources and increased energy efficiency, will be needed.

Enter energy storage

Energy storage has long been touted as the panacea for integrating renewable energy into the grid at large scale. Replacing the power generation left by Diablo Canyon’s closing will require expansive additions to wind and solar. However, more renewable energy generation will require more storage.

There are many different energy storage technologies currently available or in the process of commercialization, but each falls into one of four basic categories: chemical storage as in batteries, kinetic storage such as flywheels, thermal storage and magnetic storage.

The different technologies within each of these category can be characterized and compared in terms of their:

power rating: how much electrical current produced

energy capacity: how much energy can be stored or discharged, and

response time: the minimum amount of time needed to deliver energy.

The accompanying figures graphically compare each category of storage and how they perform on these characteristics.

With chemical storage, energy is contained in the chemical molecular structure of the storage media, such as lithium ion and lead-acid batteries.Eric Daniel FournierWith kinetic storage, energy is stored in physical (i.e., potential) movement of a mechanical system, such as flywheels, compressed air, and pumped hydro(CAES: Compressed Air Energy Storage).Eric Daniel FournierThermal storage, typically utilized in concentrated solar power applications, stores energy in the form of heat, such as molten salt. (TES: Thermal Energy Storage)Eric Daniel FournierWith magnetic storage, energy is temporarily magnetizing a conductive coil that is later discharged as an electric current, such as superconducting magnets.Eric Daniel Fournier

The key challenge that utilities are now faced with is how to integrate energy storage technologies for specific power delivery applications at specific locations.

This challenge is further complicated by the electric power transmission system and consumer behaviors that have evolved based on a energy supply system dominated by fossil fuels. Additionally, storage technologies are expensive and still developing, which makes fossil fuel generators look more economically beneficial in the short term.

Implementing storage technologies

Currently in California, energy storage is effectively provided by fossil fuel power plants. These natural gas and coal-powered plants provide steady “baseload” power and can ramp up generation to meet peaks in demand, which generally happen in the afternoon and early evening.

A single energy storage device cannot directly replace the capacity potential of these fossil fuel sources, which can generate high rates of power as long as needed.

The inability to perform a like-for-like replacement means that a more diversified portfolio strategy toward energy storage must be adopted in order to make a smooth transition to a lower carbon energy future. Such balanced energy storage portfolio would necessarily consist of some combination of:

short-duration energy storage systems that are capable of maintaining power quality by meeting localized spikes in peak demand and buffering short term supply fluctuations. These could include supercapacitors, batteries and flywheels that can supply bursts of power quickly.

Lower speed energy storage that can supply a lot of power and store a lot of energy. These systems, such as pumped hydro and thermal storage with concentrated solar power, are capable of shifting the seasonality of solar production and servicing the unique power requirements for large scale or sensitive power users in the commercial and industrial sectors.

This set of storage technologies would have to be linked up in a kind of chain, nested and tiered by end use, location and integration into the grid. Additionally, management systems will be needed to control how the storage technologies interact with the grid.

Ultracapacitors store energy as batteries do but deliver power differently – in quick bursts, making it potentially suitable for keeping the supply and demand of power in constant balance.Department of Energy

Currently without sufficient energy storage in place, utilities now use natural gas to fill in the gaps in electricity supply from renewable sources. Utilities use “peaker” plants, which are natural gas-fueled plants that can turn generation up or down to meet electricity demand, such as when solar output dips in the late afternoon and evening, while producing air pollution and greenhouse gas emissions in the process.

With natural gas consumption for electricity generation on the rise, would it be better to keep nuclear power while energy storage technologies mature? Although less polluting than coal, natural gas produces greenhouse gas emissions and has the potential to cause environmentally dangerous leaks, as seen in Aliso Canyon.

With nuclear, it is still not clear what to do with nuclear waste, and the disaster at Japan’s Fukushima nuclear power plant in 2011 highlights how catastrophically dangerous nuclear power plants can be.

Regardless of which situation you believe is best, it is clear that energy storage is the major limitation to achieving a carbon-free electricity grid.

California’s commitment to renewable energy sources has helped shift the state to using less fossil fuels and emitting less greenhouse gases. However, careful planning is needed to ensure that energy storage systems are installed to take over the baseline load duties currently held by natural gas and nuclear power, as renewables and energy efficiency may not be able to carry the burden.